Visual displays that make use of ambient light to illuminate their pixels (reflective) and that produce an image that is indefinitely stable in the absence of electrical input are often referred to as electronic paper, since they mimic some of the most advantageous properties of paper. Just like white paper that reflects and scatters incident light and does not require additional light sources for viewing the images printed upon it, electronic paper displays reflect and scatter ambient light in the white or light colored areas (often image-free areas) and absorb light in the black or dark color areas (often where the image appears). Thus, an electronic paper display can provide images that are viewable in the absence of backlight or pixel emission illumination (e.g., light emitting diode pixels). The absence of backlight makes such displays more pleasing to the eye, since the appearance of an image on such display resembles the appearance of an image on a sheet of paper. Further, since a backlight source is not required for these displays, they can be manufactured in less bulky, thin forms that may also possess some paper-like flexibility.
Electronic paper displays may also be bistable. Bistability refers to the ability of an image to remain stable in the absence of external stimuli (e.g., an applied electric potential). In bistable displays the states of individual pixels (e.g., whether the pixels are light or dark) remain intact for long periods of time when no external potential is applied to the display. Therefore, images can be stored on bistable displays for a prolonged time without the need for continuous application of power, much like images stored on paper. This makes bistable displays especially appealing for portable-display applications. Further, since power is consumed by bistable displays only when the image is changed, these displays are more economical for some applications than conventional LCD and CRT displays. In CRT displays, for instance, the image needs to be constantly refreshed. While low refresh rates can conserve some power, this often results in flickering of the display and consequent eye strain of the viewer.
The image on the electronic paper display can be changed when desired, allowing a variety of applications for such displays. In one example, such displays serve as “reusable paper” for displaying still images. In other examples, they are used to display real-time moving imagery in video applications.
The first electronic displays with paper-like properties were developed in the 1970s at Xerox's Palo Alto Research Center. These displays, often referred to as “gyricon” displays, are based on rotation of optically and electrically anisotropic spheres embedded in an elastomer. In one example of a gyricon display, each sphere is composed of negatively charged black wax or plastic on one side and positively charged white wax or plastic on the other side. Each sphere is suspended in a dielectric fluid contained within a cavity formed in a plasticized elastomer. Each sphere is free to rotate in the fluid so that it could turn with black or white side to the viewer, thus providing a pixel with a black or white appearance. When an appropriate voltage is applied to the electrodes addressing selected spheres, the spheres rotate in accordance with their dipole moment and display an image to the viewer.
Gyricon technology, however, failed to produce image quality comparable to that of images printed on paper. In particular, gyricon displays did not possess the high reflectance of white paper, therefore providing low-contrast images. Gyricon displays also had limited environmental stability, because plasticized polymer was not capable of withstanding high-temperature or high-humidity conditions. Further, only few dielectric fluids were suitable for use in gyricon displays, since dielectric fluid in gyricon was serving both as a polymer plasticizer and as a rotation media and therefore had to possess properties suitable for both of these applications.
The brightness and contrast of displayed images is primarily determined by the maximum reflectance that a display may attain. The overall reflectance of the display is influenced by the quality of optically and electrically anisotropic spheres as well as by optical properties of the material filling the gaps between individual spheres. Although in improved versions of gyricon, described in U.S. Pat. No. 5,754,332 issued to Crowley et al., these gaps are minimized by employing a closely packed monolayer of bichromal spheres, this improvement was still very far from sufficient to approach the paper-like reflectance of about 85%. Even in a closely packed monolayer there remains some elastomer or matrix material occupying gaps between the spheres, which reduces the observed reflectance of the display and, hence, the contrast and brightness of displayed images. Since gyricon technology largely relies on swelling of elastomer to encapsulate the rotating spheres, the portions of elastomer filling the interstitial regions between the spheres, typically enlarge upon swelling, and absorb a significant amount of light, even when a closely packed monolayer of spheres is employed.
U.S. Pat. No. 5,815,306 issued to Sheridon et al. describes an improved gyricon display having an “eggcrate” matrix for holding individual spheres. The matrix provides a geometrically ordered array of cavities for containing the spheres, with one sphere residing in each cavity. The matrix is used in order to align auxiliary optical devices with the spheres, so that the display can function in a light transmission mode, transmitting or obscuring the passage of light so as to create an image. Such a matrix, although useful as a holding and aligning element, does not address the problem of low reflectance in the areas between the spheres, and, consequently, does not improve the contrast and brightness of the display.
Therefore, there is a need for an electronic paper display that can provide high-contrast images. Preferably, such display will have an overall reflectance that is comparable to reflectance of paper. It should be suitable for viewing both still and moving imagery, and should allow fabrication in thin and flexible forms. In addition, such display should preferably be robust and environmentally stable, e.g., it should be capable to withstand high-temperature and high-humidity conditions.